This invention relates to temperature control indicator units. More particularly, it relates to temperature control indicator units which develop a color irreversibly upon exposure to temperatures higher than a predetermined value and, therefore, can be effectively used during the transportation or storage of various articles requiring temperature control (for example, frozen foods, chilled foods, natural flowers, drugs and the like) to judge whether the articles have been maintained within the specified temperature range or not.
Conventionally, there are a wide variety of articles which must be displayed, stored and/or transported under strict temperature control. Especially in recent years, the spread of cold chains has caused large amounts of chilled or frozen foods to appear on the market and their handling requires strict temperature control. More particularly, unless such chilled or frozen foods are maintained at or below their respective specified temperatures during the period of transportation and storage extending from their manufacture to their delivery to consumers, deterioration of the contents, putrefaction due to the propagation of bacteria, and other troubles may take place and raise a serious social problem.
Articles requiring such strict temperature control include not only chilled or frozen foods, but also natural flowers, drugs (for example, certain anticancer agents, vaccines and preserved blood), photographic chemicals, chemical reagents and the like.
Under present conditions, however, it is difficult to know whether or not articles (typically, frozen and chilled foods) requiring strict temperature control are rigidly maintained under proper temperature conditions during the period of time extending from their manufacture to their delivery to consumers. Accordingly, practicable temperature control systems are eagerly desired in related fields of art.
A number of temperature control indicators for use in such applications have already been proposed. For example, U.S. Pat. No. 3,954,011 discloses a temperature control indicator based on the fusion of a colored solid fatty acid and the capillary action of blotting paper. Moreover, Japanese Patent Publication Nos. 4319/'82 and 28559/'82 disclose another temperature control indicator based on the fact that a lipase enzyme is activated in degrees varying with the temperature and its degree of activation can be indicated by different colors with the aid of a pH indicator.